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SURVIVING THE WIRES ENVIRONMENT VIDEO1200 Hour Overhaul/Inspection; An Owner’s PerspectiveTOLER ROWLANDS N71DV


When is it time to overhaul your Bell 47 helicopter prior to reaching 1200 hours in service? To every Bell 47 owner, operator, A&P technician, etc. this question needs to be addressed whether it be from an economic or safety standpoint.

As for economics the first question an owner will ask is “How much is this going to cost me?” Speaking from experience I rarely quote a firm price to the numerous Bell 47 owner’s who contact me concerning 1200 hour overhaul/inspection. Obviously some items are a given; if a retirement item has time remaining and upon approved inspection methods is found to be airworthy then the owner is well satisfied. To the contrary should the opposite occur then be prepared to stand behind your findings, remember you (as the A&P) are accountable as a skilled professional, don’t allow an inexperienced maintenance opinion to overrule your good judgment. If the Bell 47 owner respects your work they’ll also accept the fact that a good overhaul/inspection estimate is just that (an estimate) and upon further investigation and disassembly additional cost for both labor and replacement parts should be factored into the equation.

Safety is another one of those key words we hear so often yet how many of us actually do practice it? What does it mean to be safe? Are we born with it? Is there a class offered titled “Safety 101 For A&P Technicians?”. I’m sure we could debate safety for several hours during Heli-Expo 2004 (Las Vegas) but let’s concentrate on promoting the Bell 47 instead. During Rotorfest 2003 I was asked the question “What does it mean to be safe?” I explained it this way; you are a home plate umpire (early one Saturday morning) during what every 8 year old player would subconsciously label as their “I have to win this one or no Dairy Queen for a week” World Series. On a 3-2 pitch you call out little Johnny much to the dismay of the coach and every imaginable family member that armchair quarterbacks the following Sunday afternoon. So what correlation exists between being an A&P and umpiring? It’s simple, the pitch was either a ball or a strike; not close to a ball or close to a strike, the pitch was one or the other. The same principle applies to safety around your Bell 47 whether it be during preflight, post flight, maintenance, inspection, etc.. Know the strike zone, refer to the rulebook if necessary; know your Bell 47, refer to the maintenance manual since it’s by your side. Is that chip on the tail rotor gearbox magnetic plug a pending problem or break in fuzz? Not sure? Contact a fellow Bell 47 A&P for a second opinion after emailing a digital photograph, this way the person views what you are viewing. Also consider contacting the A&P who overhauled the tail rotor gearbox and review their maintenance/overhaul procedures without rushing to a conclusion. I also like to refer to the NTSB accident investigation site paying special attention to component failures that may have lead to an accident. Remember accidents just don’t occur, there is always a chain of events leading up to the occurrence since you can’t pull it over to the curb at 90 knots and 1500 agl.

So we’re back to the question concerning 1200 hour overhaul/inspection. Speaking from experience I was curious to determine why the oil cooler was leaking on my Bell 47G2A1 (referred to as “old yeller”) during the early March of 2003. Not from a selfish standpoint but this Bell 47 would be found next to the word dependable in any dictionary; 1130.8 SMOH in 1997 and routine scheduled maintenance with a few hiccups included since purchase in April 1993. My initial plan was to operate the helicopter until 1200.0 hours (maintenance hobbs) and either trade for a larger turbine powered Bell product or sell the helicopter outright as run out. Time for accountability to appear (and the Brinks truck) as I discovered the center frame structure near the oil cooler was severely corroded thus allowing the oil cooler to vibrate during flight. Maybe severely corroded is an understatement, upon further inspection (via paint removal) I discovered the center frame structure was cracked 180 degrees, to my total disbelief (with a firm pull) the structure completely failed and I could clearly see the internal corrosion. Decision time now arrived on scene, needed to finish up power line patrol before the trees bloom but if the internal corrosion was that high then I can’t trust any part of the remaining center frame since I really enjoy the VO-435-A1F Lycoming following me around when I fly. I contacted the electric co-op to relay the news, they’re nice folks and like my flying so we agree to resume patrol flights during the upcoming winter months. Let the games begin! It’s a Sunday afternoon and the Bell 47 is resembling the Johnny Cash hit “One Piece At A Time” only in reverse since the parts were coming off versus going on.

Now Southern Illinois is a beautiful region during the springtime but can be very unpredictable when it comes to weather. Despite an unusually wet spring I was able to successfully disassemble and strip the airframe paint (outdoors) via what seemed like an endless supply of mild blast media and paint stripper. My suggestion to any Bell 47 owner is always observe the proper safety precautions/manufacturer recommendations when it comes to bead blasting and paint stripping; the blast media will disperse in all directions from both pressure (120 psi) and wind conditions while paint stripper is unforgiving on almost any contact surface. My experience would suggest staging all Bell 47 parts in a climate controlled environment free of moisture, dirt, chemicals ,etc. both prior to and after any paint removal process.

Fast forward to May 2003. Several airframe/engine accessories have returned safely upon successful completion of overhaul requirements. The owners of our local True Value would smile when I came thru their door toting box after box of alternators, magnetos, starters, actuators and camshafts for shipment.  Remarkably the biggest boxes for UPS to handle contained interior seat cushions, floor mats and vertical firewall furnishings since these items required exact duplication for reinstallation. As for heaviest it would have to be a toss up between a VO-435-A1F Lycoming crankshaft or the engine case halves, perhaps Tony (our dependable UPS package car driver) could answer this question better than I. Bottom line is whatever you schedule for shipment and regardless of where it’s going don’t skimp on packing material, although the product name escapes me I vividly  remember the mid to late 70’s luggage commercial of the caged gorilla handling a suitcase.  Now just imagine that’s your engine/rotor tachometer instrument enclosed. Fortunately I’m within 120 highway miles of an FAA Certified Repair Station in reference to magnetic particle inspection, in this case I prefer to deliver the parts myself although I place a lot of faith in the ‘big brown truck.’ What a great working relationship; drive up in the morning, unwrap all those airworthy (hopefully) main rotor transmission parts, assist the lead technician, rewrap all the parts and drive home later that afternoon.

Now to the good stuff. Based upon experience I would recommend paying particular attention to these areas (in concert with the Bell 47 Maintenance & Overhaul Instructions) while performing a 1200 hour overhaul/inspection.

If your Bell 47 has a history of spraying an initial sign of internal corrosion (due to repeated chemical exposure and daily washing) might be the 47-360-181-51 tubes that lend support to the cabin structure. The tube(s) are in close proximity to the lower door frame (both sides), perhaps their near vertical installation might lend itself to trapping both moisture and chemical residue.

Thank goodness one of our Bell 47 Helicopter Association members submitted a technical tip concerning the importance of properly inspecting, repairing, replacing and calibrating instruments and their associated piping. Some of the wiring in my Bell 47G2A1 appeared original but didn’t require replacement, I did however replace several electrical plugs, connectors, etc..  It appears over the years that flight time, vibration and atmospheric conditions have taken their toll so the statement “the best form of maintenance is preventive maintenance” would certainly apply in this situation. As A&P technicians we are limited to removal and reinstallation pertaining to instruments, prior to reinstallation always verify the helicopter operational limits (red line, green arc, yellow arc, etc.) are correct assuming an instrument repair station might have returned the instrument in airworthy condition without the required markings.

I replaced the original center frame on our Bell 47G2A1 with a freshly overhauled/repaired 47-322-241-143 center frame that was located on the “left coast.” As for total time on the original center frame I can only assume it to be in sinc with airframe total time since the helicopter maintenance records reflected nothing to the contrary. Should this statement hold true the original center frame total time would be just shy of 6000 hours, more importantly how many acres of chemicals had it supplied support for since 1967?  The corrosion was detected in numerous locations on the original center frame thus warranting its’ transport to an approved facility for further evaluation, I’ll provide a follow up story should one be appropriate.

Main rotor mast internal corrosion? Look for it and if in doubt refer to the informative story graciously submitted by a fellow member on the Bell 47 Helicopter Association website. You know the main rotor mast is one of those Bell 47 parts I prefer to pack and drive (if practical) to an FAA Certified Repair Station for inspection. Remember the gorilla and the suitcase…

There’s only one position concerning a gimbal ring and your Bell 47, refer to ASB 47-76-4 should airworthiness issues arise. My official stance is any gimbal ring assembly below a –23 makes an excellent cereal bowl; don’t hesitate to utilize 1-800-BELL-250 to confirm the –23 gimbal ring you are about to acquire is authentic.

Most FAA-PMA engine mounts are treated internally with hot linseed oil prior to delivery; primer and paint adhesion can be challenging even under ideal circumstances.

Send the oil cooler in for some well deserved maintenance, take care of it and it’ll take care of you and your freshly overhauled engine. I wouldn’t recommend reapplication of paint by the overhaul facility although they would probably do an excellent job; tape off the cooling fin region and paint it yourself. Upon reinstallation and reaching your first 100 hour inspection back flush the fin area by applying sufficient water pressure to loosen any accumulated dirt, trash, grass, etc. that might affect cooling capability. Verify the engine spark plugs/wiring harnesses are installed prior to this task since moisture might lead to corrosion where the spark plug barrel and wiring harness ‘cigarette’ meet.  If a dirty main rotor blade (leading edge) costs you and additional 1” of manifold pressure due to drag how much $ will a dirty oil cooler cost you at engine maintenance intervals? On a related note I ended up replacing both the oil pump body assembly and gear set on the Lycoming VO-435-A1F engine due to excessive wear. It appeared something had invaded both the oil pump body assembly and gear set thus inflicting an appreciable amount of damage due to rotation. Needless to say (whatever it was) there was a good possibility it also invaded the oil cooler.

Trying to reinstall new AN hardware into that freshly painted collective jackshaft? Under ideal circumstances sometimes the difference between go/no-go is cadmium plating on the new bolt. Also remember to reinstall those small cabin screws prior to refinishing, this will prevent a primer/paint combination that can make screw installation quite a challenge.

F900 torque seal is available in various colors and readily available for purchase thru numerous aircraft suppliers. I prefer to utilize green, red and white since these three colors seem more resistant to fading from environmental factors including temperature and direct sunlight. Regardless of your color preference torque seal is especially helpful when introducing a new helicopter flight candidate to their first preflight inspection.

Replace Lord mounts at normal engine change intervals if deteriorated or worn. Replace immediately if any of the following conditions exist; edges of mount clamps are striking center frame support bracket during engine operation, separation between rubber and metal has progressed approximately 120 degrees around the circumference of either inner or outer member, separation has reached a depth of ¼” the length of the Lord mount at any point, flex cracks in rubber have developed to a depth of ¼”.

Don’t forget to remove the 47-612-800-35 gasket if overhaul exchanging your engine adapter plate.

Check torque the main rotor mast nut at 25 hours time in service after initial installation.

Prior to reinstallation visually verify the carburetor throttle shaft is the proper length and will fit properly with the 47-631-360-1 adapter on the carburetor to cam box shaft assembly. On occasion even the highest quality overhaul facility can make a mistake.

Visually inspect the region where the tail rotor gearbox output shaft and 410241 seal interact. I detected wear (groove) around the output shaft circumference and opted to purchase a new output shaft. The corrective action was expensive but the alternative could be extremely costly.

In summary there are many similarities pertaining to 1200 hour overhauls/inspections despite variances in Bell 47 geographic ownership and operating conditions. Luckily for this Bell 47G2A1 owner/operator the helicopter was inspected and approved for return to service in early July 2003; it’s a rewarding feeling to know if something’s worth doing it’s worth doing right the first and only time. Good indication of a quality 1200 hour overhaul/inspection might be helicopter performance during the first 100 hours back in operation. Good indication of a quality A&P technician might be observed in those slightly torn, tattered and greasy pages (front to back) of the Bell 47 maintenance manual.


BELL 47 GIMBALS Maguirewalkaround15


The Bell position on the approved gimbal ring is quite clear and very simple.  ASB 47-76-4 was issued on 5 April 1976 and requires the installation of gimbal ring assembly 47-120-014-23 not later than 1 September 1976 regardless of the number of flight hours on the previous gimbal ring assembly.  Gimbal ring assembly 47-120-014-23 has a 4800 hour retirement life and is usable on all commercial Bell 47 model designations.  No other Bell 47 gimbal ring assembly is authorized for use by Bell Helicopter.





  1. The Bell 47 was originally certified with wood tail rotor blades, and for the sake of this discussion these blades are not considered.


  1. The first metal tail rotor hub and blade assembly used was the 47-641-058, which included blade 47-641-102 of various dash numbers.  Ordered as a blade set, the pair of blades was most recently identified as the 47-641-102-55 blade set.  This blade can be identified by a single bolt hole in the pitch horn portion of the blade root.  This set of blades installed on the hub assembly 47-641-058 which can be identified by the single 5/16 in. Delta Hinge bolt passing through the hub and tail rotor mast.  This is probably known colloquially as the early version metal blade configuration.


  1. The second iteration of the metal tail rotor blade was the 47-642-102 also of various dash numbers, which installed on hub 47-641-104.  This hub can be identified by the existence of a pivoting trunnion that attached to the mast by means of a splined joint.  Ordered as a blade set, the latest part number was 47-642-102-77 blade set.  This blade is similar in appearance and profile to the above blade, but can be differentiated by the two bolt holes on the pitch horn portion of the blade root.  Together the combination resulted in hub and blade assembly 47-641-059, this combination is probably known as the intermediate tail rotor hub and blade assembly.


  1. Service Bulletin 47-76-2 introduced the “206 style tail rotor hub and blade assembly 47-641-170,  consisting of blade 47-642-117 and hub assembly 47-641-170 both quite similar in appearance to the 206 style.  This installation is known to Bell as the improved tail rotor assembly and is probably known colloquially as the latest version tail rotor blade.  This is the only current spare configuration, and the only version approved by Bell for continued operation on any FAA certified Bell 47.


Operators who choose to continue in operation with the early version blades are accepting significant risk and have no course of litigation available resulting from any superceded tail rotor blade.  Another risk is that previously reported flight time may be fraudulent since only the improved tail rotor blade has been supplied by Bell since March 1976.  Additionally FAA AD 80-10-04 requires the removal from service of all 47-641-102 blades that are installed on a Lycoming engine-powered Bell 47.

Engine Problems


Don Maguiredon


My experience indicates that when roughness occurs at engine RPM lower than normal operating, and as evidenced by your comments and the excessive mag drop, the engine magnetos may be the cause. You make no comment about the condition of the magneto points, or the verification of the mag-engine timing. For the relatively low cost involved, you may wish to recheck the points condition and possibly replace them if there is a rough or frosty appearance to the contact finish.

I would not immediately suspect the spark plugs because if I interpret your message correctly, the engine seems smooth enough at operating RPM. Wire-electrode spark plugs perform best in VO engine installations, I have little experience with massive electrode plugs. Depending on the age and type of plugs, it is not impossible that a contribution from massive electrode plugs may result in your roughness. Additionally, I would tend to eliminate the engine harness because a fault within the magneto – spark plug wiring harness usually results in roughness at all RPM levels.

I hope you have the 1200 series Bendix-Scintilla magnetos which have always provided best service and reliability in the VO engine. A faulty magneto may not produce the desired spark intensity at low RPM while producing an apparently acceptable spark at high RPM. If the points and mag timing check out acceptable, I would suggest the magnetos as next investigation target. To verify the magnetos it may be necessary to remove them for bench test under load.


Corrosion and Proper Care


In regards to the Mast Corrosion, I’ve found two Masts with very serious internal corrosion at 1200 hr. inspections, in both cases the corrosion was beyond just surface rust. Before replacing the Masts, I sent them to the manufacture ( TEXTOOL ) in Houston TX. for inspection and/or repair. They can ,in some cases ,depending on the depth of the corrosion, hone the internal surface and the Mast can be returned to service. Fortunately the wall thickness of the 47 Mast allows a good bit of clean up, and they were able to save both. The cost of the repair was only one third the cost of a new Mast.

While on the corrosion subject, The 47 center frame & tail boom can develop internal tubular corrosion just from aging, a easy way to find if this condition exists is to use a adjustable automatic type center punch, which can be adjusted on the low-blow side and punch along the lower side of the tubing every 4 or 5 inches ( use scotch tape on the tube so as not to damage the paint) if there’s serious internal corrosion the sound will be muffled or soft, if its OK the snapping sound will be sharp & loud.


Review of Helicopter Engine Instruments paul h 2

by:  Paul D. Faltyn

Commercial Pilot and A&P Mechanic


A common misleading scenario concerning engine instrumentation among piston engine helicopter operators is that you do not repair or calibrate them unless they fail or there accuracy is suspect. All mechanically driven instruments have a ( TBO ) ” time between overhaul “, most manufacturers recommend that there instruments be overhauled between 1500 to 2500 hours in service.


Engine and Rotor Tachometers:


The engine and rotor tachometer have bearings, motors, springs, pivots, and mechanical linkages that are subject to vibration, temperature changes, and wear. As the internal lubrication break downs, spring tension changes, parts wear, the pointers can become sluggish, sticky, or loose calibration.


I would suggest that at every engine overhaul or 1200 hours the calibration should be verified and overhauled at 2500 hour intervals per most manufacturers specifications for TBO.


Tach Generators:


Tach Generators are basically an electrical motor producing power to drive the tach indicators. The generator drive shaft should be checked for excessive play or worn splines, as this can cause the shaft to shear and you will loose your indication on the instrument.


Some of the older tach generators magnets will dissipate there charge over time causing low readings on the indicator. A common indication is where you are able to maintain standard manifold pressure but unable to maintain normal RPM.


The tach generator consists of 2 bearing, an oil seal, drive shaft, and a armature. They are cheap to overhaul and will last in excess of 3500 hours, but I would strongly recommend a TBO of no more than 2500 hours.


Manifold Pressure and Oil Pressure Indicators:


These very important indicators are basically a pressure sensing element transmitting a reading to the pointer via a mechanical linkage. Most indicators will run in excess of 5000 hours without any indication of degraded accuracy or

sluggish response.


I would recommend that the gages at be tested or accuracy at engine overhaul. Most repair stations or FBO’s have ramp test equipment where the calibration can be easily verified.


One of the most neglected part of the indicating system is the pressure lines installed behind the instrument panel. Have your mechanic inspect the lines at the annual inspection. If the lines which are generally rubber hoses are hard or brittle, replace them.


Question: Why address this issue:


I recently had the opportunity to look at a Bell 47D-1 that was beautifully restored, but the owner was having a

problem with his instruments. After reviewing the logbooks we could find no indication that the tach generators or indicators were ever removed from the helicopter. After removing the rotor/tach indicator, the instruments still had

a Kollsman seal on it dated 1951.


When I removed the indicator from the panel I grabbed the manifold pressure line and it crumbled in my hands and the oil pressure hose and pitot/static hoses were not in much better condition. I also found several frayed wires behind the panel as well.


After having the tach generator, rotor-tach indicator, and manifold pressure overhauled and replacing all the hoses, the helicopter performed perfectly to specification.


Most of the Bell 47’s are flying today are at least 30 plus years old and the instrumentation will not last forever. This is a simple preventative maintenance program that can prevent unwanted head aches or unnecessary maintenance.


A helicopter operator I was associated with that operated 5 or more Bell 47’s at any given time had a strict maintenance policy of verifying the engine instrument calibration at every engine overhaul or 1200 inspection. We would generally find that the instruments required overhaul at 2500 hours and 50 percent of the time minor calibration was required at 1200 hour or engine overhaul.


Suggestion: If your helicopter is going through engine overhaul or restoration consider some preventative maintenance on your instruments. It is cheap insurance and can reduce or prevent maintenance issues. At your next inspection, have your mechanic take a look behind the instrument panel, you may be surprised what you find.


Remember, we are operating vintage helicopters and sometimes you have to take the maintenance to new levels not published in the maintenance manuals to ensure safety and reliability.